1. Field of the Invention
The present invention relates generally to a waveguide fiber having large effective area, and particularly to such a fiber designed for use over an extended wavelength range.
2. Technical Background
Dispersion shifted optical waveguide fiber having large effective area, for example that disclosed and described in U.S. Pat. No. 5,835,655, Liu et al., has been developed for use in the wavelength range which spans the low attenuation wavelength window around 1550 nm. The large effective area of the waveguide serves to limit non-linear dispersion effects that can occur in high power or high data rate systems. In U.S. Pat. No. 5,748,824, Smith, is disclosed a dispersion shifted waveguide fiber having its zero dispersion wavelength outside the operating window to limit losses in wavelength division multiplexed systems due to four wave mixing and cross phase modulation. In particular, the zero dispersion wavelength is designed to be less than the lower wavelength of the operating window so that the linear dispersion is non-zero and positive over the operating window. In such a waveguide fiber design, non-linear self phase modulation as well as four wave mixing and cross phase modulation effects are curtailed.
The demand for additional capacity has encouraged a search for waveguide index profile designs that extend the operating window into the L-band, typically defined as the wavelength range 1565 nm to 1625 nm or 1650 nm. A successful design for this extended operating range would exhibit a dispersion magnitude over the entire operating wavelength range sufficient to limit the four wave mixing and cross phase modulation effects, which become larger in systems having relatively smaller channel spacing. Such waveguide fiber designs preferably would not sacrifice performance in the C-band, typically defined as the wavelength range from 1530 nm to 1565 nm. In addition, the total dispersion slope would preferably be low enough to preclude high linear dispersion at the upper end of the wavelength range.
The problem of designing a waveguide fiber having a desired magnitude of total dispersion over at least the S (1470 nm to 1530 nm), C (1530 nm to 1565 nm), and L (1565 nm to 1650 nm) bands is addressed by the present invention. An extended S band defined as the wavelength range from 1350 nm to 1530 nm is also addressed in this application.
The following definitions are in accord with common usage in the art.
The refractive index profile is the relationship between refractive index or relative refractive index and waveguide fiber radius.
A segmented core is one that is divided into at least a first and a second waveguide fiber core portion or segment. Each portion or segment is located along a particular radial length, is substantially symmetric about the waveguide fiber centerline, and has an associated refractive index profile.
The radii of the segments of the core are defined in terms of the respective refractive indexes at respective beginning and end points of the segments. The definitions of the radii used herein are set forth in the figures and the discussion thereof.
Total dispersion of a waveguide fiber is the sum of the material dispersion, the waveguide dispersion, and the inter-modal dispersion. In the case of single mode waveguide fibers the inter-modal dispersion is zero.
The sign convention generally applied to the total dispersion is as follows. Total dispersion is said to be positive if shorter wavelength signals travel faster than longer wavelength signals in the waveguide. Conversely, in a negative total dispersion waveguide, signals of longer wavelength travel faster.
The effective area is
Aeff=2xcfx80(E2r dr)2/(E4r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with light propagated in the waveguide. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The relative refractive index percent, xcex94%=100xc3x97(ni2xe2x88x92nc2)/2ni2, where ni is the maximum refractive index in region i, unless otherwise specified, and nc is the average refractive index of the cladding region.
The term xcex1-profile refers to a refractive index profile, expressed in terms of xcex1 (b)%, where b is radius, which follows the equation,
xcex94(b)%=xcex94(bo)(1xe2x88x92[|bxe2x88x92bo|/(b1xe2x88x92bo)]xcex1),
where bo is the point at which xcex94(b)% is maximum, b1 is the point at which xcex94(b)% is zero, and b is in the range bi less than b less than bf, where delta is defined above, bi is the initial point of the xcex1-profile, bf is the final point of the xcex1-profile, and xcex1 is an exponent which is a real number.
The pin array bend test is used to compare relative resistance of waveguide fibers to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two attenuation measurements. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. The waveguide fiber is caused to pass on opposite sides of adjacent pins. During testing, the waveguide fiber is placed under a tension just sufficient to make the waveguide conform to a portion of the periphery of the pins. The test pertains to macro-bend resistance of the waveguide fiber.
Another bend test referenced herein is the lateral load test. In this test a prescribed length of waveguide fiber is placed between two flat plates. A #70 wire mesh is attached to one of the plates. (The market code #70 mesh is descriptive of screen made of wire having a diameter of 0.178 mm. The screen openings are squares of side length 0.185 mm.) A known length of waveguide fiber is sandwiched between the plates and a reference attenuation is measured while the plates are pressed together with a force of 30 newtons. A 70 newton force is then applied to the plates and the increase in attenuation in dB per unit length is measured. This increase in attenuation is the lateral load attenuation of the waveguide. The test pertains to the micro-bend resistance of the waveguide fiber.
A first aspect of the present invention is an optical waveguide fiber, typically a single mode optical waveguide fiber, having a core region surrounded by a clad layer. The core region and the clad layer are characterized by respective refractive index profiles. The core region is divided into a central segment, a first annular segment surrounding the central segment, and a second annular segment surrounding the first annular segment. Each segment has a refractive index profile and an inner and an outer radius. The core segments are configured to provide a waveguide fiber having a positive total dispersion in the range of 1.0 ps/nm-km to 16.0 ps/nm-km over a wavelength range extending from 1470 nm to 1625 nm.
An extension of this aspect (aspect extension) of the invention is one in which the wavelength range of operation is 1450 nm to 1625 nm. The total dispersion (sometimes referred to as chromatic dispersion) is in the range of 0.1 ps/nm-km to 18 ps/nm-km over the range of operation, and the zero dispersion wavelength is less than 1450 nm. In a preferred embodiment of the aspect extension, the total dispersion at 1525 nm is no greater than 11 ps/nm-km.
At 1550 nm the waveguide of this first aspect is characterized by an effective area greater than 60 xcexcm2, preferably greater than 70 xcexcm2, and an attenuation less than 0.25 dB/km, preferably less than 0.20 dB/km. In the aspect extension the effective area is greater than 70 xcexcm2 and preferably greater than 80 xcexcm2. The attenuation at 1550 nm of the aspect extension is less than 0.20 dB/km. At 1600 nm the first aspect of the invention has effective area greater than 70 xcexcm2, preferably greater than 80 xcexcm2, and more preferably greater than 85 xcexcm2. At 1600 nm, the aspect extension has an effective area greater than 90 xcexcm2 and more preferably greater than 95 xcexcm2. The attenuation at 1600 nm is less than 0.25 dB/km, preferably less than 0.22 dB/km, and more preferably less than 0.20 dB/km. The attenuation of the aspect extension at 1600 nm is less than 0.21 dB/km and preferably less than 0.20 dBkm. In addition, the pin array bend loss and lateral load bend loss at 1550 nm of a waveguide made in accordance with this first aspect of the invention are less than 12 dB and less than 1.0 dB/km, respectively. The pin array bend loss at 1600 nm is less than 16 dB. In the aspect extension the pin array bend loss is less than 10 dB and the lateral load bend loss is less than 1.3 dB/km and preferably less than 1.0 dB/km.
An embodiment of the waveguide fiber in accord with the first aspect of the invention has a central segment of relative index percent, xcex94o, 0.50%xe2x89xa6xcex94oxe2x89xa61.0%, inner radius zero, and outer radius ro, 3.5 xcexcmxe2x89xa6roxe2x89xa65.0 xcexcm. A first annular segment has relative index percent, xcex941, 0.01%xe2x89xa6xcex941xe2x89xa60.1%, and outer radius r1, 5.5 xcexcmxe2x89xa6r1xe2x89xa610 xcexcm. A second annular segment has relative index percent, xcex942, 0.15%xe2x89xa6xcex942xe2x89xa60.35%, center radius r2c, 7.0 xcexcmxe2x89xa6r2cxe2x89xa611.0 xcexcm, and width w2, 0.8 xcexcmxe2x89xa6w2xe2x89xa62.5 xcexcm.
An embodiment of the waveguide fiber in accord with the aspect extension has a central segment of triangular shape and relative index percent, xcex94o, 0.50%xe2x89xa6xcex94oxe2x89xa60.75%, inner radius zero, and outer radius ro, 4.0 xcexcmxe2x89xa6roxe2x89xa65.5 xcexcm. A first annular segment has relative index percent, xcex941, 0.01%xe2x89xa6xcex941xe2x89xa60.04% and outer radius r1, 6 xcexcmxe2x89xa6r1xe2x89xa68 xcexcm. A second annular segment has relative index percent, xcex942, 0.25%xe2x89xa6xcex942 less than 0.40%, center radius r2c, 6.5 xcexcmxe2x89xa6r2cxe2x89xa69.0 xcexcm, and width w2, 1.0 xcexcmxe2x89xa6w2xe2x89xa62.5 xcexcm. This embodiment may have an optional centerline depression having a relative index percent on centerline of about 0.3% and a half relative index radius no greater than about 0.5 xcexcm. The term half relative index is defined in the detailed description.
In another embodiment, the waveguide fiber in accordance with the first aspect of the invention has an index depression on its centerline. The depression exhibits a minimum relative index of no less than zero and a radius, measured at the minimum index, of no greater than 1 xcexcm. In this embodiment, the core parameters of the waveguide fiber are such that a central segment has relative index percent, xcex94o, 0.50%xe2x89xa6xcex94oxe2x89xa61.0% and outer radius ro, 3.0 xcexcmxe2x89xa6roxe2x89xa65.0 xcexcm. A first annular segment has relative index percent, xcex941, 0.01%xe2x89xa6xcex941xe2x89xa60.1% and outer radius r1, 6.0 xcexcmxe2x89xa6r1xe2x89xa69 xcexcm. A second annular segment has relative index percent, xcex942, 0.15%xe2x89xa6xcex942xe2x89xa60.50%, center radius r2c, 7.0 xcexcmxe2x89xa6r2cxe2x89xa611.0 xcexcm, and width w2, 1.0 xcexcmxe2x89xa6w2xe2x89xa63.0 xcexcm.
In an embodiment of the aspect extension of the invention, the waveguide fiber has an index depression on its centerline. The depression exhibits a minimum relative index in the range of 0.1% to 0.3% and a radius, measured at the minimum index, in the range of 0.5 to 1 xcexcm. In this embodiment, the core parameters of the waveguide fiber are such that a central segment has relative index percent, xcex94o, 0.50%xe2x89xa6xcex94oxe2x89xa60.8% and outer radius ro, 3.5 xcexcmxe2x89xa6roxe2x89xa64.5 xcexcm. A first annular segment has relative index percent, xcex941, 0.015%xe2x89xa6xcex941xe2x89xa60.35% and outer radius r1, 7 xcexcmxe2x89xa6r1xe2x89xa68 xcexcm. A second annular segment has relative index percent, xcex942, 0.30%xe2x89xa6xcex942xe2x89xa60.40%, center radius r2c, 8.0 xcexcmxe2x89xa6r2cxe2x89xa69.0 xcexcm, and width w2, 1.0 xcexcmxe2x89xa6w2xe2x89xa62.0 xcexcm.
The shape of the profile of the respective segments in any of the embodiments set forth herein are selected from the group consisting of a step, a rounded step, a trapezoid, a rounded trapezoid, and an xcex1-profile wherein xcex1, 0.1xe2x89xa6xcex1xe2x89xa650. The triangular profile is known to be an xcex1-profile having xcex1=1.
In yet a further embodiment of the waveguide in accordance with the first aspect of the invention, the core region includes a third annular segment having a negative relative index, where, as is stated above, the reference index is taken as the average index of the clad layer. The addition of this third annular segment provides a waveguide fiber having an effective area at 1550 nm greater than 70 xcexcm2 and preferably greater than 80 xcexcm2, and attenuation at 1550 nm less than 0.25 dB/km and preferably less than 0.20 dB/km. The waveguide fiber made in accordance with this embodiment also has pin array bend loss less than 5 dB at 1550 nm and less than 8 dB at 1600 nm. The lateral load bend loss at 1550 nm is less than 1.38 dB/km and at 1600 nm less than 2.55 dB/km. The effective area at 1600 nm of this embodiment is greater than 90 xcexcm2 and preferably greater than 95 xcexcm2 and the 1600 nm attenuation is less than 0.25 dB/km and preferably less than 0.22 dB/km.
A waveguide made in accordance with this embodiment has a third annular segment of relative index xcex943, xe2x88x920.15%xe2x89xa6xcex943xe2x89xa6xe2x88x920.45%, center radius r3c, 12.0 xcexcmxe2x89xa6r3cxe2x89xa617.0 xcexcm, and width w3, 2.5 xcexcmxe2x89xa6w3xe2x89xa65.0 xcexcm. This embodiment has a central segment of relative index percent, xcex94o, 0.50%xe2x89xa6xcex94oxe2x89xa61.0% and outer radius ro, 4.0 xcexcmxe2x89xa6roxe2x89xa66.0 xcexcm. The first annular segment has relative index percent, xcex941, 0.01%xe2x89xa6xcex941xe2x89xa60.10%, and outer radius r1, 7.5 xcexcmxe2x89xa6r1xe2x89xa611 xcexcm. The second annular segment has relative index percent, xcex942, 0.35%xe2x89xa6xcex942xe2x89xa60.6%, and outer radius r2, 10 xcexcmxe2x89xa6r2xe2x89xa615 xcexcm.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.